The present invention relates to an isocyanate-terminated prepolymer which is useful as a binder for lignocellulose-containing materials.
Composite wood products such as particle board or flake board are produced by blending a binder composition with wood flakes or other comminuted lignocellulosic materials, forming this blended composition into a mat and compressing the mat between heated platens to set the binder and bond the flakes together in densified form. Conventional manufacturing processes are generally carried out at temperatures of from about 150 to about 205.degree. C. These processes also generally require that the moisture content of the lignocellulosic material be between 2 and 8% before it is blended with the binder.
Binder compositions which have been used in making such composite wood products include phenol formaldehyde resins, urea formaldehyde resins and isocyanates. See, for example, James B. Wilson's paper entitled, "Isocyanate Adhesives as Binders for Composition Board" which was presented at the symposium "Wood Adhesives--Research, Applications and Needs" held in Madison, Wis. on Sep. 23-25, 1980, in which the advantages and disadvantages of each of these different types of binders are discussed.
Isocyanate binders are commercially desirable because they have high adhesive and cohesive strength, flexibility in formulation, versatility with respect to cure temperature and rate, excellent structural properties, the ability to bond with lignocellulosic materials having high water contents, and no formaldehyde emissions. The disadvantages of isocyanates are difficulty in processing due to their high reactivity, adhesion to platens, lack of cold tack, high cost and the need for special storage.
Isocyanate prepolymers are among the isocyanate materials which have been used in binder compositions to solve various processing problems. U.S. Pat. No. 4,100,328, for example, discloses isocyanate-terminated prepolymers which improve product release from a mold. U.S. Pat. No. 4,609,513 also discloses a binder which is an isocyanate-terminated prepolymer to improve product release. U.S. Pat. No. 5,179,143 discloses a binder composition in which a particular type of isocyanate prepolymer is used to improve adhesiveness at room temperature.
A major processing difficulty encountered with isocyanate binders is the rapid reaction of the isocyanate with water present in the lignocellulosic material and any water present in the binder composition itself. One method used to minimize this difficulty is to use only lignocellulosic materials which have a low moisture content (i.e., a moisture content of from about 3 to about 8%). This low moisture content is generally achieved by drying the cellulosic raw material to reduce the moisture content. Such drying is, however, expensive and has a significant effect upon the economics of the process. Use of materials having low moisture contents is also disadvantageous because panels of the product composite material tend to absorb moisture and swell when used in humid environments.
One approach to resolving this problem is disclosed in U.S. Pat. No. 4,546,039. In the disclosed process for bonding lignocellulose-containing raw materials, a lignocellulosic material having a moisture content of up to 20% is coated with a prepolymer based on a diphenylmethane diisocyanate mixture. This prepolymer has a free isocyanate group content of about 15 to about 33.6% by weight and a viscosity of from 120 to 1000 mPa.s at 25.degree. C. This prepolymer is prepared by reacting (1) about 0.05 to about 0.5 hydroxyl equivalents of a polyol having a functionality of from 2 to 8 and a molecular weight of from about 62 to about 2000 with (2) one equivalent of a polyisocyanate mixture containing (a) from 0 to about 50% by weight of polyphenyl polymethylene polyisocyanate and (b) about 50 to about 100% by weight isomer mixture of diphenlymethane diisocyanate containing 10-75% by weight of 2,4'-isomer and 25-90% by weight of 4,4'-isomer.
U.S. Pat. No. 5,002,713 discloses a method for compression molding articles from lignocellulosic materials having moisture contents of at least 15%, generally from 15 to 40%. In this method, a catalyst is applied to the lignocellulosic material, a water resistant binder is then applied to the particles and the coated particles are then compression shaped at a temperature of less than 400.degree. F. to form the desired composite article. The catalyst used is a tertiary amine, an organometallic catalyst or a mixture thereof. The binder may be a hydrophobic isocyanate such as polymeric diphenylmethane diisocyanates, m- and p-phenylene diisocyanates, chlorophenylene diisocyanates, toluene diisocyanates, toluene triisocyanates, triphenylmethane triisocyanates, diphenylether-2,4,4'-triisocyanate and polyphenol polyisocyanates. The catalyst is included to ensure that the isocyanate/water reaction is not slowed to such an extent that the pressing time necessary to produce the molded product is significantly increased.
Pressing of wafer board, oriented strand board, and parallel strand lumber using steam injection and a conventional polymeric diphenylmethane diisocyanate (MDI) is known. Such processes yield a product having satisfactory physical properties. However, during the process, MDI is emitted from the press in the excess steam. The finished boards also emit MDI upon removal from the press.
It would be advantageous to provide an isocyanate-based binder which (1) is sufficiently water-tolerant that it may be used with lignocellulosic raw materials having moisture contents of from about 10% and higher, (2) allows shorter press times without requiring a catalyst, and (3) reduces isocyanate emissions during and subsequent to production of wood composites therefrom.